This invention relates to gas flow circuit breakers in general and more particularly to an improved arc quenching arrangement for a blast piston circuit breaker.
A blast piston circuit breaker in which a check valve is installed in a flow channel leading through a portion of the insulating material to the quenching gap is disclosed in U.S. Application Ser. No. 454,544 filed Mar. 25, 1974 and assigned to the same assignee as the present invention. In such a circuit breakers sulfurhexafluoride SF.sub.6, which is typically used as an insulating medium in encapsulated high voltage installations, is often used at the same time as the quenching medium. Gas flow circuit breakers of this nature are typically used in high voltage installations for quenching arcs in circuits carrying high currents. As disclosed in the aforementioned application a plurality of check valves in appropriately designed flow canals can be used to obtain the required quenching. Alternatively, the quenching or flow channel can also be designed in the form of an annular channel having a fairly large number of resilient reeds each serving the purpose of a check valve.
In such a device the pressure in the quenching medium which is required to quench the arc is generated during the motion of the circuit breaker by a blasting piston device in the quenching arrangement. In the aforementioned application a nozzle quenching system is formed by the check valves in which the pressure generated by the arc in the quenching gap is utilized for self-blasting of the arc. The check valves prevent hot quenching gasses from flowing back into the high pressure chamber i.e. the compression chamber in the blasting piston circuit breaker. Through this arrangement the excess pressure generated by the arc in the quenching gap cannot act upon the piston. As a result the circuit breaker need only be designed for a limited gas pressure.
However, in such an arrangement after the check valves in the flow channels are closed by the pressure generated at the arc in the quenching gap during interruption of the circuit, and particularly when large currents are interrupted, the gas in the high pressure section is further compressed due to motion of the piston which continues. This work of compression must, of course, be supplied by a drive mechanism.
It has been discovered that, due to the coupling between the movable electrode and the piston, an excessively high compression can be built up in the high pressure chamber between the time when the check valve closes and the time when it is re-opened. Because the electrode and piston are tied together, piston motion is determined only by the required electrode velocity. Because of these factors the drive mechanism of a quenching system of this nature must be designed to take into account the high compression in the high pressure chamber which may result during such operation. However, such high pressures are not required and not utilized during the quenching process.
In view of these problems it is the object of the present invention to limit the pressure generated in the high pressure chamber to a value required to quench the arc once the back pressure has been relieved and the check valve re-opened.